When a ship is cruising, a boundary layer of water (or sea water) is produced around the hull of the ship due to the viscosity of the water that is a fluid. The flow velocity of the water at the boundary layer with respect to the ship's hull is zero at the hull's surface and tends to increase rapidly as the distance from the hull's surface increases. This is one of the most important causes of the hull resistance since the frictional-resistance of water affects the ship's hull.
Thus, in recent years, research has been pursued for improving the propelling performance of a ship by reducing the frictional-resistance affecting the ship's hull. One possible countermeasure being studied is the micro-bubble-propelling method in which air bubbles are injected from the hull's surface into the boundary layer of the submerged portion of the ship's hull to reduce the frictional-resistance affecting the hull's surface by covering the submerged portion of the ship's hull with air bubbles.
As a means for embodying such a propelling method employing air bubbles, it is known to eject pressurized air, generated by an air supplying device such as an air pump, into water, from a ship's outer surface so as to distribute the air bubbles around the hull.
A method for producing air bubbles in water is also known in which, for example, pressurized air is ejected from a nozzle, a slit formed in a ship's hull, or an air conduit (such as a pipe perforated with a plurality of holes).
However, in such a conventional technique for generating air bubbles by ejecting pressurized air simply at the periphery of the ship's hull, the air ejection ports should be formed everywhere at the periphery of the ship's hull so as to cover as wide an area of the hull as possible with air bubbles; alternatively, the air ejection ports should be formed at least over the entire area from the upper portion to the lower portion of the submerged portion of the hull in order to cover the submerged portion of the hull from the shallower area to the deeper area. As a result of this, static head at the air ejection portion is relatively high, and thus, the energy consumption for ejecting the pressurized air is large, offsetting the saving of cruising energy from the reduction of frictional-resistance.
Furthermore, the conventional method in which air is ejected from the known nozzle, slit, or air conduit (such as a pipe perforated with holes) could not produce micro-air-bubbles having very small diameters. Consequently, it was not possible to effectively hold the bubbles in the boundary layer surrounding the ship's hull, and thus, effective frictional-resistance-reducing effects could not be obtained. Alternatively, in order to avoid producing such problems, it was necessary to provide air ejection portions over nearly the entire periphery of the hull so as to cover as wide an area of the hull as possible with air bubbles, thus requiring large amounts of energy for ejecting the air.
There are also several reports of research regarding the behavior of air-bubbles used for reducing the frictional-resistance of a navigating body. For example, Madavan, et al., studied a direct model of frictional-resistance reduction, using a model in which the apparent local density and the local coefficient of viscosity, which vary due to the presence of air bubbles in the liquid phase, are used as parameters of the mixing-length theory (Madavan J L, Merkle C L, Deutsch S; 1985; "Numerical Investigation into the Mechanisms of Microbubble Drag Reduction"; Trans ASME; vol.107, p370-377). Marie proposed a model in which the thickness of a viscous sublayer varies because of the presence of air bubbles, using the apparent local density and the local coefficient of viscosity similar to the model of Madavan (Marie J L; 1987; "A Simple Analytical Formulation for Microbubble Drag Reduction" Physico Chemical Hydrodynamics; vol. 8-2, p213-220). However, taking into account the fact that the apparent local coefficient of viscosity used by Madavan, et al., and by Marie employs Einstein's model, the models seem to be more appropriate for the case of analyzing a medium, referred to as a "suspension", in which particles finer than two-phase flow are dispersed. The size of the suspension is generally 0.01-100 .mu.m. It should be further examined whether Einstein's model is applicable to the flow containing air bubbles having diameters of approximately 1 mm (which will be discussed in the present invention). Furthermore, since these theoretical investigations do not discuss the assumption about the void fraction, these theories do not appear to be realistic. Thus, problems relating to the determination of the amount of air bubbles to be ejected or relating to effective use of the ejected air bubbles remain in these techniques, the problems resulting from the difficulties in exactly understanding the behavior of air bubbles ejected into a water stream.
Therefore, a primary object of the present invention is to provide a method for reducing the frictional-resistance of a ship's hull in which effective reduction in frictional-resistance of the hull can be surely realized and to provide a frictional-resistance reducing ship using such method.
Another object of the present invention is to provide a method and an apparatus which can surely produce the micro-air-bubbles required for carrying out the present invention.
Still another object of the present invention is to provide a frictional-resistance reducing ship which can substantively eliminate the need for an air-bubble-generating device by utilizing micro-bubbles contained in stem-broken-waves produced at the stem portion of a ship during cruising.
Still another object of the present invention is to provide a method for analyzing the behavior of ejected air bubbles, which is required to embody the method for reducing the frictional-resistance, this object including the following concrete objects:
clarifying the behavior of air bubbles in a water stream;
improving the understanding of the distribution, feed amount, and amount of presence of air bubbles in the boundary layer;
making it possible to calculate the frictional-resistance reduction effect due to the supply of air bubbles into the boundary layer; and
making it possible to calculate the supplying position and the supplying amount of air bubbles in response to the hull form, thus improving the cost-benefit ratio of the expenditure of energy of the frictional-resistance reduction ship due to the ejection of air bubbles.